First, try reading some erotica. (If you like AIF, you probably won't mind this research.) Links for writing good erotica can be found in the links section.
If you look at bad erotica (not hard to find), you'll see that the stories often suffer from just too much sex, with paragraph after paragraph of cookie-cutter people with superhuman stamina having emotionless, repetitive sex. Now, we have nothing against superhuman stamina and lots of sex. But to hold the player's attention, the sex ought to be interesting.
Now look at the good erotica. You'll notice first the quality plots and characterizations. But you'll also notice just the right amount of sexual tension, sexual situations, and of course, really hot sex. And the same things that make other fiction interesting make sex interesting: originality, detail, and variety. In short, quality in sex is IMO more important than quantity (although, to be frank, quantity is fun, too). Or, to quote H_Slave, "Quality has its own type of quantity."
Don't overdo your sex scenes. Sometimes graphic descriptions of sex, genitals, and bodily fluids can be fun. But many players will find descriptions of clothing, other body parts, and atmosphere even more erotic. Earl of Kent writes, "I'd rather have less explicit, but better written, sex scenes as opposed to more explicit but downright laughable ones." Of course you get to choose whether to refer to a woman's womanhood, vagina, pussy, or something else. And you probably will (and should) use more than one term, depending on the particular scene. Just remember that your choice of words matters.
Mycophile suggests, "AIF is hardly an optimal medium for communicating the subtle physicalities of sex, but it's perfect for set, setting, and circumstance." Various ways of making sex in AIF interesting are discussed in the following sections. They require good writing, good coding, and most of all, imagination. In turn, they provide differing amounts of eroticism, mimesis, and plot. Balancing all these things will create good AIF.
One of the things that distinguishes GNA and HI is the sheer diversity of sexual situations (all accomplished, by the way, even though NewKid stuck with pretty vanilla sex). Good AIF changes the *circumstances* of sex, not just which tab goes in which slot. This is of course one of the main things that makes RLS interesting even when doing the same thing with the same person - changing circumstances. The other advantage of using interesting circumstances for your erotic scenes is that you have the challenge and opportunity to write an interesting and unique plot that will lead to those scenes.
Sex can be different when you're lying, standing, or sitting. Sex can also change depending on how dressed or undressed each partner is. Sex can obviously involve putting different things in different other things. Many more methods for creating sexual diversity are listed below. Each will be useful in certain circumstances, and each can add to diversity; but probably overusing any one tool will make a game tedious. (E.g., it would be tough to write 20 sex scenes with the same character in the same location where only the player/NPC sex position changed. Educational, maybe, but tough.)
"Incomplete" sex can also help build a story, and to build a player-NPC relationship. Several AIF's (Emy, STX, etc.) depend on the player kissing a character at one meeting, fondling them later in the story, etc. If you're in a literary mood, you can also think of this as foreshadowing. And flirtation definitely makes a subsequent sex scene more believeable. Including sexual encounters that don't go all the way is probably one of the most important characteristics of good AIF, because it can add so much in terms of plot, interactivity, and eroticism. Also, used sparingly, coitus interruptus can add some of the humorous frustration common in IF, as long it we get some coitus uninterruptus eventually.
Myco pointed out while reading an early draft that the guide emphasized sex, not the wider field of erotica. A scene with two people can be very erotic without anyone having an orgasm. A scene with one person can be erotic, even if it's not masturbation. It's all about good writing.
Here are just a few possibilities for the background plot, the player-NPC relationship, and the NPC's role and mindset for a sex scene. The player and NPC(s) could be shooting a porno movie, cheating on someone, escaping a terrible monster, or just celebrating New Year's. They could have just met at a bar or a club, they could be married (to each other?), one might work with or for the other, they might hate each other, or they might be rivals in a contest. The NPC might be an active partner, the initiator, or just lying there. She may be thankful, angry, or just plain horny. Changing any one of these aspects will make the scene totally different, in subtle or obvious ways.
Adding different reasons for the sex itself adds variety, too. Gratitude, seduction, and general sluttiness have been done, and there's nothing wrong with returning to a good motif. Less common motivations - like blackmail (HI's Laura) or even BJD's natural consequence of coed naked trampolining - add variety too. In retrospect, one of the sex scenes in RC may be a seduction with ulterior motives; that adds to re-playability.
All of these different possibilities show that the writer must carefully craft the whole scene, and the scenes before and after it. A good writer will also change the writing (and dialogue, if any) within the erotic scene to reflect the atmosphere. Sex in a haunted house might involve lots of groping in the dark, the characters trying to reassure each other that everything's alright. Sex on a beach might be carefree, with lots of giggling and tickling. Sex on your tenth anniversary might be slow and loving. Or fast and loving. Or really boring. (Don't knock a boring sex scene! You might be able to contrast it with a really hot sex scene later in the game with the same or a different partner. Or imagine starting the game with not-so-good sex: the player would be instantly drawn in, but want to play more to get real satisfaction.)
Similarly, the writing and dialogue should reflect the player's and NPC's personalities and their current emotions. The main idea here is that sex scenes shouldn't be generic. If you could cut and paste your sex scenes to another place/character/time in the game, then you're probably not worrying enough about details.
Another possibility which is less common in AIF is to actually let the NPC start something. Why not have an NPC who wraps herself around the player as soon as he walks into the room? Legend has it that in RLS, sometimes women even initiate, say, oral sex, without the man telling them to! While surprise sex might remove interactivity temporarily, it might offer quite an erotic payoff. And again, this would just be one weapon in the author's arsenal.
Even larger orgies ought to be possible, although unless they're implemented as cut scenes, they become exponentially more difficult to code. Also, they may not be alive, but Moist's sex toys probably count in this category, too. And the list of objects that can be used for sex is pretty extensive, Visit's food products being just one example.
I love voyeurism. I put it in all my games. Voyeurism is practically essential to the main process in STX. It also is well suited to (A)IF. It makes for great puzzles. You can't just walk down the street and see people having sex (unless you live in certain areas of Miami....) You gotta do some work to get into a position to see. You then see. You often then learn other things, or pick up useful items in the process of getting into position to spy on others, or even get caught and are propelled into a whole new storyline as a result.
Clothes are the most obvious ways to do this. Clothes can conceal or reveal, they can be transparent, they can be opened, closed and removed. (Although this all requires some complex coding!) But don't forget the NPC's facial expressions and other body language; whether she's sitting, standing or laying down; any objects she may be interacting with (a chair, a pet bird, a radio). And of course there's her current interaction with the player, if any! And don't forget smells and other senses in your NPC descriptions!
NewKid points out that character descriptions in particular can be difficult to write and code well. Because they may change so radically depending on the player's and NPC's circumstances, it's hard to write bug-free code that also yields good prose in every circumstance. For example, the player may not have met Esmerelda yet when he meets her twin Willemina, so as much as you want to compare them, you need to be careful. A more subtle case might involve a case where, when the characters begin having sex, the NPC is either standing or sitting. It might be as easy as writing code to say that if the NPC is sitting, then write "Brunhilde springs from her chair, like a flower wilting in reverse." But then again, it might not. Unfortunately, this needs to be solved on a case by case basis. You (or your beta testers) will have to actually look at the text in as many situations as possible.
Remember that details about a place help mimesis, and may help (or hinder) the sex as well. A hard floor will make sex more uncomfortable than on a bed. A closet hook or rod is something to hang on to. Furniture is good for leaning, sitting, or lying on. Reading erotica and playing AIF should give you hints here.
Speaking of talking, don't forget that good dialogue can make sex much more fun. And erotic, of course. Dialogue - from "Yes!" to whole conversations - is pretty common in RLS and erotica, and should be in AIF too. Again, the dialogue should depend on the player, the NPC, and the current situation. Sex is a fun time to throw in new plot wrinkles or unexpected information. The simplest example would be an NPC telling you something after sex, because you've won her over to your side. But how about an NPC who calls out the wrong name during sex? Or a spy who can't keep secrets when she's aroused? It's also a great time for character development: what a woman says may let you know that she's nervous, confident, feels inferior, obsessed with the color blue, whatever. But aside from all the plot and character development, dialogue is also just another way to make this sex scene different from the other fourteen sex scenes in your game.
This is especially important because some people find certain acts much more or less interesting than others. So please, think of these scenes as a cheap way to introduce non-linearity into your games. It's fine if you want to give extra points to people who do every possible sex act, but they don't all need to be mandatory. In short, let me have sex the way I want to.
There are a couple of implementation details, of course. The current tools and examples are designed from the male perspective. But it shouldn't be too hard to change, especially with more modern game designing systems. Women can initiate and receive oral sex the same way men do. And "fuck guy" would have a pretty obvious meaning. It might take some careful designing... but careful design leads to better games, anyway.